The present invention relates to a phase type spatial light modulator for use in an optical information processor in a visual apparatus for an industrial robot or the like, which optical information processor performs image processings such as filtering and extraction of features in a spatial frequency region of an input image, a processing for identifying, from a plurality of input patterns, one or more input patterns coincident with a specific standard pattern, etc.
An arrangement and operation of a known optical information processor described in Japanese Patent Laid-Open Publication No. 2-132412 (1990) are described with reference to FIG. 11, hereinbelow. In FIG. 11, a collimator lens 2 converts a ray from a laser diode 1 into collimated rays and a liquid crystal display 13 displays an image shot by a television (TV) camera 3. The liquid crystal display 13 is disposed at a front focal surface of a lens 6 having a focal length f1 and a memory 8 stores halographic data of a Fourier transformation computer. A liquid crystal display 14 for displaying the data stored in the memory 8 is disposed at a rear focal surface of the lens 6. Meanwhile, the liquid crystal display 14 is also disposed at a front focal surface of a lens 11 having a focal length f2 and a photoelectric converter 12 is disposed at a rear focal surface of the lens 11.
Interrelation among the above mentioned constituent elements of the known optical information processor and operation of the known optical information processor are as follows. A pattern of a target subject displayed by the liquid crystal display 13 is irradiated by a coherent ray from the laser diode 1. This image of the target subject is optically converted by the lens 6 and a Fourier transformed image of the target subject is formed on the liquid crystal display 14. At this time, a Fourier transformed image of a standard pattern stored in the memory 8 is displayed by the liquid crystal display 14. Thus, output light from the liquid crystal display 14 is formed by an optical product of two Fourier transformed images of the target subject and the specific standard pattern. Since the liquid crystal display 14 is disposed at the front focal surface of the lens 11, this image is subjected to reverse Fourier transformation by the lens 11. When the Fourier transformed images of the target subject and the standard pattern have coincided with each other, a bright point is produced on the rear focal surface of the lens 11 and is detected by the photoelectric converter 12. Thus, the known optical information processor performs optical correlation processing in which an optical filter based on the computer hologram displayed on the liquid crystal display 14 functions as a matched filter.
FIG. 12 shows an actual construction of the liquid crystal displays 13 and 14 of FIG. 11. In FIG. 12, a liquid crystal cell 16 is formed by twisted nematic (TN) liquid crystal and is interposed between polarizers 15 and 17 whose transmission axes are parallel to each other. The transmission axis of the polarizer 15 is substantially parallel to a direction of alignment of liquid crystal molecules at an input side of the liquid crystal cell 16.
FIG. 13 shows relations among directions of alignment of liquid crystal molecules of the liquid crystal cell 16 and directions of the transmission axes of the polarizers 15 and 17. In FIG. 13, reference numeral 50 denotes a direction of alignment of liquid crystal molecules at an output side of the liquid crystal cell 16, reference numeral 51 denotes a direction of alignment of liquid crystal molecules at an input side of the liquid crystal cell 16 and reference numeral 52 denotes the direction of the transmission axis of the polarizer 15. Furthermore, reference numeral 53 denotes the direction of the transmission axis of the polarizer 17 and reference numeral 54 denotes a twist angle of the liquid crystal cell 16.
When no voltage is applied to the liquid crystal cell 16, direction of polarization of output light from the polarizer 15 is rotated through nearly 90.degree. by the liquid crystal cell 16 and thus, the output light from the polarizer 15 is intercepted by the polarizer 17. When a voltage is applied to the liquid crystal cell 16, twist of the TN liquid crystal cell 16 is eliminated and rotation of polarized light is reduced, thereby resulting in an increase of the light component in the direction of the transmission axis of the polarizer 17. Therefore, the liquid crystal displays 13 and 14 function as amplitude modulating elements.
FIG. 14 shows a configuration of the computer hologram displayed on the liquid crystal display 14 shown in FIG. 11. Since the liquid crystal display 14 is the amplitude modulating element, it becomes possible to not only modulate the phase component of input light by selecting on a cell 60 the position of pixel groups 55 to 58 transmitting light therethrough, but also to modulate amplitude component of the input light by controlling the number of the selected pixel groups. Therefore, the liquid crystal display 14 is capable of acting as a medium for displaying the computer hologram and can execute optical information processings such as pattern matching.
However, in the above described arrangement of the known optical information processor, since the liquid crystal display 14 is the amplitude modulating element, phase component of input light is modulated by changing spatial position of the light transmitting pixel groups. Hence, the cell should be arranged such that a plurality of pixels correspond to one sample point. Namely, utilization efficiency of pixels in the liquid crystal cell is low and the number of pixels which can be subjected to batch processing is lessened.